How do I open the doors?

The 747-400 and 747-400F use custom code for the door logic, as such
the default SHIFT-E commands will not work.

Go to the PMDG menu within MSFS and then the General | Keyboard
Assignment option. There you will find a category listing for Doors 747.
Assign your desired key combinations for each of the doors and exit the
menu. You will now be able to open each door individually according to
the key combinations you assigned.